fix(core): Ensure ComponentFixture does not duplicate error reporting from FakeAsync (#60104)

atscott · kirjs · commit 962b59b14e9d · 2025-02-26T11:56:18.000-05:00
When using `fakeAsync`, if errors happen inside the Angular zone, they are also handled by the fakeAsync machinery. Rethrowing errors from the `NgZone.onError` subscription is not appropriate in this case because it results in the error being thrown twice. https://github.com/angular/angular/blob/db2f2d99c82aae52d8a0ae46616c6411d070b35e/packages/zone.js/lib/zone-spec/fake-async-test.ts#L783-L784 https://github.com/angular/angular/blob/db2f2d99c82aae52d8a0ae46616c6411d070b35e/packages/zone.js/lib/zone-spec/fake-async-test.ts#L473-L478 PR Close #60104
diff --git a/packages/core/test/component_fixture_spec.ts b/packages/core/test/component_fixture_spec.ts
@@ -21,7 +21,9 @@ import {
 import {
   ComponentFixtureAutoDetect,
   ComponentFixtureNoNgZone,
+  fakeAsync,
   TestBed,
+  tick,
   waitForAsync,
   withModule,
 } from '@angular/core/testing';
@@ -372,6 +374,37 @@ describe('ComponentFixture', () => {
     expect(() => fixture.detectChanges()).toThrow();
   });
 
+  it('should not duplicate errors when used with fake async', fakeAsync(() => {
+    @Component({
+      template: '<button (click)="doThrow()">a</button>',
+    })
+    class Throwing {
+      doThrow() {
+        throw new Error('thrown');
+      }
+    }
+    TestBed.configureTestingModule({
+      providers: [
+        {
+          provide: ErrorHandler,
+          useClass: class {
+            handleError(e: unknown) {
+              throw e;
+            }
+          },
+        },
+      ],
+    });
+    const fix = TestBed.createComponent(Throwing);
+    try {
+      fix.nativeElement.querySelector('button').click();
+      tick();
+      fail('should have thrown');
+    } catch (e) {
+      expect((e as Error).message).toMatch('thrown');
+    }
+  }));
+
   describe('errors during ApplicationRef.tick', () => {
     @Component({
       template: '',
diff --git a/packages/core/testing/src/component_fixture.ts b/packages/core/testing/src/component_fixture.ts
@@ -122,6 +122,17 @@ export class ComponentFixture<T> {
       this.subscriptions.add(
         this._ngZone.onError.subscribe({
           next: (error: any) => {
+            // The rethrow here is to ensure that errors don't go unreported. Since `NgZone.onHandleError` returns `false`,
+            // ZoneJS will not throw the error coming out of a task. Instead, the handling is defined by
+            // the chain of parent delegates and whether they indicate the error is handled in some way (by returning `false`).
+            // Unfortunately, 'onError' does not forward the information about whether the error was handled by a parent zone
+            // so cannot know here whether throwing is appropriate. As a half-solution, we can check to see if we're inside
+            // a fakeAsync context, which we know has its own error handling.
+            // https://github.com/angular/angular/blob/db2f2d99c82aae52d8a0ae46616c6411d070b35e/packages/zone.js/lib/zone-spec/fake-async-test.ts#L783-L784
+            // https://github.com/angular/angular/blob/db2f2d99c82aae52d8a0ae46616c6411d070b35e/packages/zone.js/lib/zone-spec/fake-async-test.ts#L473-L478
+            if (typeof Zone === 'undefined' || Zone.current.get('FakeAsyncTestZoneSpec')) {
+              return;
+            }
             throw error;
           },
         }),
diff --git a/packages/zone.js/lib/zone-spec/fake-async-test.ts b/packages/zone.js/lib/zone-spec/fake-async-test.ts
@@ -780,6 +780,8 @@ class FakeAsyncTestZoneSpec implements ZoneSpec {
     targetZone: Zone,
     error: any,
   ): boolean {
+    // ComponentFixture has a special-case handling to detect FakeAsyncTestZoneSpec
+    // and prevent rethrowing the error from the onError subscription since it's handled here.
     this._lastError = error;
     return false; // Don't propagate error to parent zone.
   }

Original file line number	Diff line number	Diff line change
`@@ -780,6 +780,8 @@ class FakeAsyncTestZoneSpec implements ZoneSpec {`
`780`	`780`	`targetZone: Zone,`
`781`	`781`	`error: any,`
`782`	`782`	`): boolean {`
	`783`	`+ // ComponentFixture has a special-case handling to detect FakeAsyncTestZoneSpec`
	`784`	`+ // and prevent rethrowing the error from the onError subscription since it's handled here.`
`783`	`785`	`this._lastError = error;`
`784`	`786`	`return false; // Don't propagate error to parent zone.`
`785`	`787`	`}`